Question

A beam of light parallel to the principal axis of a concave mirror converges to a point. What is the point called? Draw adiagram to illustrate it.

Answer 1

When a beam of light passes parallel to the principal axis of a concave mirror the beam converges, which meet at the "Focus"
 
The image formed is real, and reduced size.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Not just for concave mirror, for any mirror or lens a beam of light coming from infinity and parallel to the principal axis will fall on the focal of the mirror/lens

Answer 2

A beam of light parallel to the principal axis of a concave mirror converges to a point.The point is known as focus of the mirror.

The rule states that a ray proceeding parallel to the principle axis after reflection pass through the principle focus in case of concave mirror.

The distance from pole to focus of mirror is known as focal length and for concave mirror focal length is positive and is half the length of centre of curvature for mirror.

When an object is kept at infinite distance image is formed at focus and is very small in size and is real and erect.

The diagram is as below in attachment to illustrate:--